From the standpoint of energy saving and reduction of environmental burdens, research toward the development of a shortened bake curing step and reduced operation steps has been actively conducted in the field of automotive body coating and industrial product coating.
For example, after electrodeposition coating, intercoating and top coating are typically provided in sequence, and bake curing has conventionally been performed following each of the intercoating and top coating. However, in an effort to reduce the operation steps in automotive body coating, there has been progress in research aimed at shortening the baking step by performing a single baking step for both the intercoating and top coating after the top coating is provided. This coating method comprising a shortened baking step is becoming the mainstream method.
With respect to shortening the bake curing step, there exists an increasing demand for lowering temperatures and shortening the time required for performing the step.
As a coating composition that meets the demand for low-temperature, short-time curing, hydroxyl/isocyanate-crosslinked coating compositions have been considered to be a strong candidate in view of cost efficiency. Hydroxyl/isocyanate-crosslinked coating compositions are obtained by crosslinking hydroxy-containing resins using a polyisocyanate compound as a crosslinking agent and curing the crosslinked resins. Thus, such compositions have been researched so far.
However, the prior art has been unsatisfactory in maintaining pot life (usable time, i.e., storage performance) in an attempt to improve the curability at low temperatures in a short period of time, and it has also been difficult to satisfy the requirement for an excellent appearance in automobile body coating and other kinds of coating.
Patent Literature 1 discloses as a coating composition excellent in curability at low temperatures a coating composition comprising as essential components a pyrazole-blocked polyisocyanate compound that contains per molecule two or more tertiary isocyanate groups blocked by a pyrazole compound, and a hydroxy-containing resin having a specific hydroxyl value and a specific weight average molecular weight. However, such a coating composition has been unsatisfactory in curability under short-time curing conditions.
Reference 2 discloses as a coating composition excellent in curability at low temperatures a one-package low-temperature-curing color enamel coating composition comprising: a resin component containing a polyester polyol having a specific hydroxy value and an acrylic polyol having a specific hydroxy value; a crosslinking agent containing a blocked isocyanate as an essential component and melamine as an optional component; a blocked isocyanate dissociation catalyst comprising at least a dicarboxylate of dibutyltin; and a pigment. However, this coating composition has been unsatisfactory in curability under low-temperature baking conditions.
Patent Literature 3 discloses a curable coating composition comprising a resin composition for preparing a coating composition and a (blocked) polyisocyanate compound, wherein the resin composition comprises a polyol (A1), a polyether polyol (B), and a solvent (C), wherein the polyol (A1) has a ring structure. Patent Literature 2 further discloses another curable coating composition comprising an organometallic compound and an acidic substance in addition to the aforementioned curable coating composition.
This coating composition is excellent in curability at low temperatures in a short period of time in an embodiment where the coating composition comprises an organometallic compound and an acidic substance; however, the pot life is unsatisfactory, and the finished appearance of the resulting coating film is also sometimes unsatisfactory.